Adsorption and Absorption

Adsorption and Absorption

Solid surfaces have the ability to attract the contacting species due to free valency or residual force on them.

For example: charcoal adsorbs ammonia, silica gel adsorbs water., charcoal adsorbs colorants from sugar.

These examples prove that adsorption is a surface phenomenon. In contrast to adsorption, absorption is a bulk phenomenon i.e. the adsorbate molecules are distributed throughout the adsorbent.

• Adsorbent is the material on which adsorption takes place.

• Adsorbed substance is called an adsorbate.

• The surface of separation of the two phases where the concentration of adsorbed molecule is high is known as interface.

• In adsorption, if the concentration of a substance in the interface is high, then it is called positive adsorption. If it is less, then it is called negative adsorption.

• The process of removing an adsorbed substance from the surface is called desorption.

• The gaseous molecules like He, Ne,O₂ ,N₂ ,SO ₂ and NH₃ and solutions of NaCl or KCl can be adsorbed by suitable adsorbents. These are referred as adsorbates.

• Silica gel and metals like Ni,Cu, Pt, Ag and Pd and certain colloids can act as adsorbents.

Characteristics of adsorption

1. Adsorption can occur in all interfacial surfaces i.e. the adsorption can occur in between gas-solid, liquid solid, liquid-liquid, solid- solid and gas-liquid.

2. Adsorption is a spontaneous process and it is always accompanied by decrease in free energy. When ∆G reaches zero, the equilibrium is attained.

We know, ∆G = ∆H - T ∆S where ∆G is Change in Free energy.

∆H is Change in enthalpy and ∆S = Change in entropy.

3. When molecules are adsorbed, there is always a decrease in randomness of the molecules. ie., ∆S < 0, and T∆S is negative. Hence, adsorption is exothermic.

Adsorption is a quick process whereas absorption is a slow process.

M.C. Bain introduced a term ‘sorption’ to represent the simultaneous adsorption and absorption. T. Graham used a term occlusion for sorption of gases on metal surfaces.

Types of adsorption

Adsorption is classified as physical adsorption and chemical adsorption, depending on the nature of forces acting between adsorbent and adsorbate. In chemical adsorption, gas molecules are held to the surface by formation of chemical bonds. Since strong bond is formed, nearly 400 KJ / mole is given out as heat of adsorption.

Examples

• Adsorption of O₂ on tungsten, Adsorption of H₂ on nickel, Adsorption of ethyl alcohol vapours on nickel.

In physical adsorption, physical forces such as van der waals force of attraction, dipole dipole interaction, dispersion forces etc., exist between adsorbent and adsorbate. As these forces are weak, heat of adsorption is low, hence physical adsorption occurs at low temperatures.

Examples

(a) Adsorption of N₂ on mica.

(b) Adsorption of gases on charcoal.

The following table 10.1 illustrates the distinction between chemical and physical adsorption.

Table 10.1 Distinction between chemical and physical adsorption

Chemical adsorption or Chemisorption or Activated adsorption

• It is very slow

• It is very specific depends on nature of adsorbent and adsorbate.

• Chemical adsorption is fast with increase pressure, it can not alter the amount.

• When temperature is raised chemisorption first increases and then decreases.

• Chemisorption involves transfer of electrons between the adsorbent and adsorbate.

• Heat of adsorption is high i.e., from 40- 400kJ/mole.

• Monolayer of the adsorbate is formed.

• Adsorption occurs at fixed sites called active centres. It depends on surface area

• Chemisorption involves the formation of activated complex with appreciable activation energy.

Physical adsorption or van der waals adsorption or Physisorption

• It is instantaneous

• It is non-specific

• In Physisorption, when pressure increases the extent of adsorption increases.

• Physisorption decreases with increase in temperature.

• No transfer of electrons

• Heat of adsorption is low in the order of 40kJ/mole.

• Multilayer of the adsorbate is formed on the adsorbent.

• It occurs on all sides.

• Activation energy is insignificant.

Factors affecting adsorption

The adsorption is well understood by considering the various factors affecting it.

Qualitatively, the extent of surface adsorption depends on the following factors

(i) Nature of adsorbent

(ii) Nature of adsorbate

(iii) Pressure

(iv) Concentration at a given temperature.

1. Surface area of adsorbent:

As the adsorption is a surface phenomenon it depends on the surface area of adsorbent. i.e., higher the surface area, higher is the amount adsorbed.

2. Nature of adsorbate

The nature of adsorbate can influence the adsorption. Gases like SO₂ ,NH₃ ,HCl and CO₂ are easily liquefiable as they have greater van der waal’s force of attraction. On the other hand, permanent gases like H ₂ ,N₂ and O₂ can not be liquefied easily. These permanent gases are having low critical temperature and adsorbed slowly, while gases with high critical temperature are adsorbed readily.

3. Effect of temperature

When temperature is raised chemisorption first increases and then decreases. whereas physisorption decreases with increase in temperature.

4. Effect of pressure:

chemical adsorption is fast with increase in pressure, it can not alter the amount of adsorption. In Physisorption the extent of adsorption increases with increase in pressure.